Facile synthesis of nanostructured LiMnPO₄ as a high-performance cathode material with long cycle life and superior rate capability

Abstract

Lithium manganese phosphate (LiMnPO₄) has been considered as an alternative to lithium iron phosphate (LiFePO₄) for next-generation Li-ion battery cathodes because of its higher working voltage. However, facile preparation methods for high-performance LiMnPO₄ are still lacking. In this work, we propose a facile route to prepare nano-LiMnPO₄ (30–50 nm) by using citric acid (CA) as a surfactant. The addition of a small amount of CA in the precursor leads to an obvious size reduction of LiMnPO₄. After carbon-coating, nano-LiMnPO₄ exhibits excellent rate capability and long cycle life at high rates because of the small size and uniform/thin carbon coating. At a high rate of up to 20C (3.4 A g⁻¹), LiMnPO₄/C can still deliver a high discharge capacity of 96.6 mA h g⁻¹. LiMnPO₄/C also exhibits long cycle life with ∼70% capacity retained after 500 cycles at 10C. The excellent electrochemical performance of LiMnPO₄/C makes it an attractive cathode in high-power and high-energy Li-ion batteries.

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1. Introduction

LiMPO₄ (M = Fe, Mn, Co) with an olivine-type structure has gained wide interest as a new cathode material for Li-ion batteries since the first report on LiFePO₄ by Goodenough and co-workers in 1997.¹ In these olivine-type materials, LiFePO₄ has now realized practical applications in electric vehicles (EVs) because of its environmental friendliness, low cost and structural stability.^2,3 Compared with LiFePO₄, LiMnPO₄ (LMP) could provide a larger energy density with its higher redox potential of Mn²⁺/Mn³⁺ (4.1 V vs. Li/Li⁺) compared to Fe²⁺/Fe³⁺ (3.45 V vs. Li/Li⁺).^4–6 However, LiMnPO₄ exhibits a rather lower electrochemical activity than LiFePO₄ due to its intrinsically lower electronic and ionic conductivity,^7,8 the structural instability of the MnPO₄ phase,^9,10 and the larger volume change between LiMnPO₄ and MnPO₄.¹¹ In addition, Mn³⁺ in the charged state undergoes Jahn–Teller distortion.^12,13 In recent years, a great effort has been made to improve the electrochemical activity of LiMnPO₄ through cation substitution, size decrease, optimized carbon coating, etc.¹⁴ Cation substitution has been found to be an effective measure to activate LiMnPO₄ and stabilize the delithiated phase.¹⁵ However, the substitution should be controlled at a low level to maintain the high energy density of LiMnPO₄.^16–24

Size decrease is another useful method to enhance the electrochemical activity of LiMnPO₄. Oh et al. synthesized LiMnPO₄ using a spray-pyrolysis/ball-milling route.²⁵ LiMnPO₄ of 10–50 nm could deliver high capacities of 158 mA h g⁻¹ at 0.05C and 126 mA h g⁻¹ at 1C after coating with a uniform carbon layer. Recent work has shown that nano-engineering could remarkably improve the electrochemical performance of LiMnPO₄.^18,26–42 Since Yang et al. first reported the direct synthesis of LiFePO₄ using a hydrothermal method,⁴³ hydrothermal/solvothermal routes have been widely used to prepare LiMPO₄ (M = Fe, Mn) with a nanostructure.⁴⁴ The size and morphology of LiMnPO₄ can be easily regulated by controlling the synthetic conditions (temperature, time, reactant concentration/ratio, etc.) and using different solvents or surfactants.^{26–29,35–39} The work by Qin et al. indicated that the morphology of LiMnPO₄ can be controlled by simply adjusting the pH value.²⁷ The obtained LiMnPO₄ nanoplates could yield high capacities of 149 mA h g⁻¹ at 0.1C and 90 mA h g⁻¹ at 1C after graphene coating. Hong et al. synthesized LiMnPO₄ nanorods by setting the volume ratio of ethylene glycol (EG) and water at 11 : 1.³⁷ The carbon-coated LiMnPO₄ could deliver a high capacity of 110 mA h g⁻¹ at 10C and a capacity retention of ∼94.5% after 100 cycles at 0.5C.

It is generally accepted that nano-engineering is a practical strategy to realize the high performance of LiMnPO₄ materials. Nevertheless, a challenge still remains to find a facile preparation method for nanosized LiMnPO₄. For the solvothermal synthesis of LiMnPO₄, the reaction of H₃PO₄ + 3LiOH + MnSO₄ = LiMnPO₄ + Li₂SO₄ + 3H₂O is usually adopted. The morphology of LiMnPO₄ was found to depend greatly on the molar ratios of H₃PO₄/LiOH/MnSO₄.^27,39 Actually, acidity plays a critical role in determining the morphology of LiMnPO₄ in the reactions. In this work, nanostructured LiMnPO₄ was prepared using a facile solvothermal route in EG/H₂O mixed solvent with citric acid (CA) as a surfactant. The results showed that the addition of a small amount of CA leads to an obvious size decrease and a considerable performance improvement of LiMnPO₄. The LiMnPO₄/C granules of 30–50 nm could deliver high capacities of 147.9, 113.0 and 96.6 mA h g⁻¹ at 1C, 10C and 20C, respectively. The capacities can be retained at 89.1 and 80.5 mA h g⁻¹ after 500 cycles at 1C and 10C, respectively. The intrinsic mechanism for performance enhancement was also investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). This work provides a facile method to realize high performance of LiMnPO₄ materials.

2. Experimental section

2.1 Preparation of LiMnPO₄ and LiMnPO₄/C

LiMnPO₄ was prepared using a facile solvothermal route via the reaction H₃PO₄ + 3LiOH + MnSO₄ = LiMnPO₄ + Li₂SO₄ + 3H₂O in EG/water mixed solvent.³⁹ The molar ratio of H₃PO₄ to LiOH to MnSO₄ in the precursor is 1 : 3 : 1. During the preparation of the MnSO₄ solution in EG/water, the desired amount of CA (1–7 mmol) was added. The reaction products are named LMP-x, where x represents the amount of CA used (in the unit of mmol). For example, when 1.0 mmol of CA was used, the product is named LMP-1.0. The carbon coating procedure was conducted according to previous work.³⁹ For simplicity, LiMnPO₄/C uses the same name as the corresponding LiMnPO₄ sample.

2.2 Material characterization

X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max-2550pc powder diffractometer (Cu K_α, λ = 0.1541 nm) to analyze the crystalline structure of LiMnPO₄. The morphology and microstructure of LiMnPO₄ and LiMnPO₄/C were checked using scanning electron microscopy (SEM) on an S-4800 microscope and transmission electron microscopy (TEM) on a JEM 2100F microscope. The carbon content in the LiMnPO₄/C samples was measured using a Flash EA 1112 tester. In this equipment, the solid carbon can be combusted into gaseous CO₂ in a rapid and dynamic mode. The carbon content can be determined by analyzing the amount of CO₂ with chromatography. The Brunauer–Emmett–Teller (BET) specific surface area of LiMnPO₄ was calculated based on the N₂ absorption/desorption isotherms using a Quantachrome Autosorb-1 analyser.

2.3 Electrochemical measurements

The electrochemical performance of LiMnPO₄/C was measured using CR2025-type coin cells on a Neware battery cycler (Shenzhen, China). The electrode was made by homogeneously mixing LiMnPO₄/C, acetylene black and polyvinylidene fluoride in a mass ratio of 7 : 2 : 1. The active material (LiMnPO₄/C) loading was around 2 mg. The cell assembly was conducted in a glove box filled with pure Ar. For the cells, metallic Li foils were used as the counter electrodes, 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 by volume) was used as the electrolyte, and Celgard 2300 microporous membranes were used as the separators. The cells were tested with a constant-current–constant-voltage (CC–CV) protocol. The cells were first galvanostatically charged to 4.5 V at different current rates, then held at 4.5 V for 1 h, and galvanostatically discharged to 2.0 V. The charge and discharge processes use the same current rate. The current density was calculated based on the total mass of LiMnPO₄ and carbon for the LiMnPO₄/C composites. The specific capacity of LiMnPO₄/C is normalized by the mass of LiMnPO₄ and 1C is defined as 170 mA g⁻¹. CV tests were conducted at 2.0–4.5 V (vs. Li/Li⁺) with a scan rate of 0.1 mV s⁻¹ on a VersaSTAT3 electrochemistry workstation (Princeton Applied Research). EIS tests were carried out on the VersaSTAT3 workstation using an ac voltage of 10 mV amplitude in a frequency range of 10 mHz to 100 kHz. The EIS measurements were performed in the lithiated state of LiMnPO₄ after the cells were rested for several hours. The electrochemical tests were performed at room temperature.

3. Results and discussion

Fig. 1 gives the XRD patterns of the solvothermal products with different amounts of CA in the precursors. The phase purity of the LiMnPO₄ samples was confirmed by comparing with the standard diffraction peaks of LiMnPO₄ (space group pnmb, JCPDS card no. 33-0804). Note that the relative intensity of the diffraction peaks changes with an increase in the CA amount, suggesting changes in the morphology and size of LiMnPO₄. The sharp peaks suggest high crystallinity in the LiMnPO₄ samples even though they were prepared at low temperature.

Fig. 1 XRD patterns of LiMnPO₄ prepared with different CA amounts in the precursors.

The morphology of the solvothermal products was characterized using SEM. The LiMnPO₄ exhibits a spindle-like shape when it was prepared with a CA-free precursor (Fig. S1†). The size of the spindle-like LiMnPO₄ is around 200 nm and the BET specific surface area is 32.2 m² g⁻¹. As seen in Fig. 2a, the shape and size of LiMnPO₄ show a remarkable change when a small amount of CA (CA/MnSO₄ molar ratio is 1/10) was added in the precursor. The obtained LiMnPO₄ demonstrates a plate-like shape with a size below 100 nm. The size of LiMnPO₄ can be further reduced to 30–50 nm by increasing the CA amount (CA/MnSO₄ molar ratio is 3/10). The LiMnPO₄ exhibits an irregular granule shape with the BET surface area increasing to 53.8 m² g⁻¹ (Fig. 2b). The size of LiMnPO₄ shows a trend to increase when the amount of CA was further increased (Fig. 2c–f). At a CA/MnSO₄ molar ratio of 7/10, the LiMnPO₄ crystallizes into a plate-like shape again and the surface area decreases to 45.7 m² g⁻¹ (Fig. 2f). Even so, the plate-like LiMnPO₄ still has a smaller size than the spindle-like one, suggesting that CA does play a critical role in reducing the size of LiMnPO₄. This size decrease, in turn, enhances the electrochemical performance of LiMnPO₄, which will be discussed later.

Fig. 2 SEM images of LiMnPO₄ prepared with different CA/MnSO₄ molar ratios in the precursors: (a) 1/10, (b) 3/10, (c) 7/20, (d) 2/5, (e) 1/2 and (f) 7/10.

Fig. 3 presents TEM images of the pristine LMP-3.0 and carbon-coated LMP-3.0. As seen in Fig. 3b, the pristine LMP-3.0 exhibits an irregular shape with a size of 30–50 nm, agreeing with the SEM observations. The morphology of the sample was retained after carbon coating as shown in Fig. 3c. The high-resolution TEM (HRTEM) image in Fig. 3d indicates that the LiMnPO₄ is well crystallized. The lattice spacings of 0.21 and 0.36 nm are related to the (112) and (111) planes of LiMnPO₄. The surface of LiMnPO₄ is uniformly coated by a layer of carbon with a thickness of ∼1 nm. As a result, size decrease of LiMnPO₄ has been realized through a facile solvothermal route using CA as the surfactant. The mechanism is schematically illustrated in Fig. 4. The adsorption of CA on the surface of LiMnPO₄ inhibits its continuous growth during the solvothermal reaction. The LiMnPO₄ grows into an irregular shape due possibly to the different adsorption ability of CA on the different crystalline planes of LiMnPO₄. However, excess CA will greatly change the acidity of the solution, leading to the formation of plate-like LiMnPO₄, similar to the case of H₃PO₄ where excess H₃PO₄ also results in the crystal growth of LiMnPO₄.³⁹

Fig. 3 TEM images of (a, b) pristine and (c, d) carbon-coated LMP-3.0.

Fig. 4 Schematic illustration of the CA-induced restrained growth of LiMnPO₄ crystals.

Electrochemical tests were performed on three LiMnPO₄/C samples with different sizes to reveal the size dependence of the electrochemical performance. Fig. 5a gives the first charge–discharge curves of the LiMnPO₄/C samples at 0.05C. The capacities of LiMnPO₄/C were calculated normalized to the mass of LiMnPO₄. As seen in the figure, these samples exhibit high electrochemical activity at a low current rate, delivering high discharge capacities (164.5 mA h g⁻¹ for LMP-3.0, 163 mA h g⁻¹ for LMP-3.5, 161 mA h g⁻¹ for LMP-4.0). Specially, LMP-3.0 yields the highest discharge capacity of 164.5 mA h g⁻¹, which is close to the theoretical capacity of LiMnPO₄ (170 mA h g⁻¹). The highest capacity of LMP-3.0 is closely correlated with it having the smallest crystal size which maximizes the utilization of active material. For LiMnPO₄ materials, irreversible capacities in the first cycle are usually observed, which is attributed to the passivation of the electrolyte and electrode at high potentials.^37,45 The irreversible capacities of LMP-3.0, LMP-3.5 and LMP-4.0 are 17.5, 19 and 24 mA h g⁻¹, respectively. Fig. 5b shows the CV scans of the samples at 0.1 mV s⁻¹. LMP-3.0 displays obviously stronger and sharper current peaks than LMP-3.5 and LMP-4.0, indicating that it has the fastest electrochemical reaction kinetics due to its crystal size being the smallest.

Fig. 5 (a) Voltage profiles and (b) CV plots of carbon-coated LiMnPO₄.

Fig. 6 compares the rate capability of the LiMnPO₄/C samples at current rates of 0.1–20C. The charge and discharge processes of the cells were performed at the same current rates in the rate capability tests. Note that the plateau length decreases with an increase in the current rate. The polarization also increases with an increase in the current density. LMP-3.0 shows the best rate capability among the three samples. The discharge capacities of LMP-3.0 are 158.6, 152.3, 147.9, 140.0 and 126.1 mA h g⁻¹ at 0.1C, 0.5C, 1C, 2C and 5C, respectively. At 10C and 20C, this sample can still deliver high capacities of 113.0 and 96.6 mA h g⁻¹, respectively. The superior rate capability of LMP-3.0 can be ascribed to its small crystal size and uniform/thin conductive carbon layer, making rapid electron and Li-ion transport possible. The LMP-3.0 sample shows a slower capacity decrease with current density than LMP-3.5 and LMP-4.0 especially at high current densities, implying that the crystal size does exert an obvious effect on Li-ion transport at the electrode/electrolyte interface and in bulk crystals.

Fig. 6 Rate capability of LiMnPO₄/C: (a) LMP-3.0, (b) LMP-3.5 and (c) LMP-4.0.

Fig. 7 demonstrates the cycling stability of the LiMnPO₄/C samples. As seen in Fig. 7a, LMP-3.0, LMP-3.5 and LMP-4.0 can deliver high initial discharge capacities of 147.2, 142.8 and 142.6 mA h g⁻¹ at 1C, which can be maintained at 89.1, 84.3 and 75.1 mA h g⁻¹ after 500 cycles. LMP-3.0 exhibits the best cycling stability with a capacity retention of over 60% after 500 cycles at 1C. Even after 500 cycles at 10C, this sample can still keep a discharge capacity of 80.5 mA h g⁻¹, with a retention of around 70%. Although the cycling stability of LiMnPO₄/C has been enhanced by various strategies in recent work,^{25–30,32,33,35–37,41,42} there are few reports on LiMnPO₄/C that can sustain 500 cycles at such a high current density (10C). It should be stressed that the charge and discharge in this work were performed at the same current rate. The outstanding cycling stability of LMP-3.0 could be due to the uniform carbon coating which prevents Mn dissolution,^25,34,42,45 and the small crystal size which alleviates the volume strain between LiMnPO₄ and MnPO₄.^11,42,46,47 In contrast, the large-sized spindle-like LiMnPO₄ exhibits low capacity and poor cycling stability (Fig. S2†). The low capacity and poor cycling stability of spindle-like LiMnPO₄ can be attributed to a low Li-ion diffusion rate with the insufficient utilization of active material and poor carbon coating for large-sized LiMnPO₄ particles. Table 1 compares the rate capability and cycle life of some LiMnPO₄/C composites from this work and others. The data summarized in Table 1 represent the best data on LiMnPO₄/C materials reported to date. Of note is that both the rate capability and cycle life of our LMP-3.0 sample are among the best when we compare the charge/discharge mode, applied current rate and cycle number comprehensively. We propose that the outstanding electrochemical properties of our LiMnPO₄/C can be attributed to the small size and uniform carbon coating, which lead to rapid electron and Li-ion transport and the easy release of the lattice strain upon repeated cycling. Carbon coating also led to remarkably improved electrochemical performance in other cathode materials such as LiNi_0.8Co_0.1Mn_0.1O₂ and LiNi_0.5Mn_1.5O₄ by stabilizing the structure and supplying conducting channels.^48,49 In addition, the ultrathin carbon layer facilitates Li-ion diffusion across the electrode/electrolyte interface with enhanced electrode kinetics. It should be noted that the LiMnPO₄/C in our work was prepared using a facile solvothermal route using small amounts of inexpensive and nontoxic citric acid.

Fig. 7 Cycling stability of LiMnPO₄/C at (a) 1C and (b) 10C.

Table 1 Comparison of the electrochemical performance of LiMnPO₄/C in this work with others^a

Sample and preparation method	Cycling stability				Rate capability				Reference
	Current density	Initial capacity (mA h g^−1)	Cycle number	Capacity retention	Current density
					Capacity (mA h g^−1)
a Note: SR = solvothermal reaction, SSR = solid state reaction, BM = ball milling, HT = high temperature, HP = high pressure, ch = charge, dis = discharge, LMP = LiMnPO4, SDBS = sodium dodecyl benzene sulfonate, G = graphene, CTAB = hexadecyltrimethyl ammonium bromide, PMMA = polymethyl methacrylate, PVP = polyvinylpyrrolidone, and OA = oleic acid.
LMP-3.0, SR with CA	10C-ch, 10C-dis	117.6	100/500	83%/68%	1C-ch, 1C-dis	5C-ch, 5C-dis	10C-ch, 10C-dis	20C-ch, 20C-dis	This work
					147.9	126.1	113.0	96.6
LMP, spray pyrolysis and BM	0.05C-ch, 0.5C-dis	∼140	50	94.2%	0.05C-ch, 1C-dis		0.05C-ch, 2C-dis	0.05C-ch, 10C-dis	25
					126		107	∼60
LMP plates, SR with SDBS	0.05C-ch, 0.05C-dis	147	50	93%	0.1C-ch, 1C-dis		0.1C-ch, 2C-dis	0.1C-ch, 5C-dis	26
					∼110		∼95	∼70
LMP/G, SR + spray drying	1C-ch-dis, 2C-ch-dis, 5C-ch-dis	∼90	60	75%	1C-ch, 1C-dis		2C-ch, 2C-dis	5C-ch, 5C-dis	27
					90		∼75	64
LMP grains, SR with CTAB	0.05C-ch, 0.05C-dis	153	110	95.4%	0.05C-ch, 1C-dis		0.05C-ch, 5C-dis	0.05C-ch, 10C-dis	28
					128		111	92
LMP, precipitation + BM	0.05C-ch, 0.2C-dis	∼135	45	90.5%	0.05C-ch, 1C-dis		0.05C-ch, 5C-dis	0.05C-ch, 10C-dis	30
					120		90	61
Porous LMP, PMMA template	—				0.1C-ch, 1C-dis		0.1C-ch, 6C-dis	0.1C-ch, 10C-dis	31
					154		129	110
LMP sheets, SR with HT, HP and PVP	0.2C-ch, 0.2C-dis	157	50	93.6%	5C-ch, 5C-dis		10C-ch, 10C-dis	20C-ch, 20C-dis	32
					119		93	63
LMP granules, SSR with OA	0.1C-ch-dis, 0.2C-ch/0.5C-dis	122	50 + 50	97.5% + 96.4%	0.05C-ch, 5C-dis		0.05C-ch, 10C-dis	0.05C-ch, 20C-dis	33
					95.7		87.1	60.1
LMP plates, SR with CTAB	0.2C-ch, 1C-dis	130.3	500	92.7%	1C-dis		5C-dis	10C-dis	35
					127.6		93.8	69.2
LMP, SR	0.5C-ch, 0.5C-dis	138	100	91.5%	1C-ch, 1C-dis		5C-ch, 5C-dis	10C-ch, 10C-dis	36
					∼135		118	106
LMP rods, SR	0.5C-ch, 0.5C-dis	144.5	100	94.5%	1C-ch, 1C-dis		5C-ch, 5C-dis	10C-ch, 10C-dis	37
					137		∼125	110
LMP flakes, SR + sintering	0.5C-ch, 0.5C-dis	∼135	200	>95%	1C-ch, 1C-dis		5C-ch, 5C-dis	10C-ch, 10C-dis	40
					130		110	92
LMP, BM + SSR	1C-ch, 1C-dis	128	200	94%	0.1C-ch, 1C-dis		0.1C-ch, 2C-dis	0.1C-ch, 5C-dis	42
					>120		∼105	∼60

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EIS tests were used to reveal the different electrode kinetics among the three samples. As seen in Fig. 8a, the Nyquist plots of the LiMnPO₄/C samples are constructed with a high-frequency semicircle and a low-frequency sloping line. The plots were fitted using an equivalent circuit (see inset in Fig. 8a). In the circuit, R_e denotes the electrolyte and ohmic resistance, R_i and Q₁ are related to the contact resistance of the active material with the current collector and the related capacitance, respectively, R_ct and Q₂ represent the charge transfer resistance and double-layer capacitance, respectively, and Z_w is the Warburg impedance related to Li-ion bulk diffusion.^50–52 As shown in Table 2, LMP-3.0 exhibits a much lower R_ct value compared with LMP-3.5 and LMP 4.0 although they have similar R_i values. The low R_ct value means that there are rapid electrochemical reaction kinetics on the electrode/electrolyte interface, which is closely related to the uniform/thin conductive carbon layer and large specific surface area of LMP-3.0.

Fig. 8 (a) Nyquist plots and equivalent circuit of LiMnPO₄/C and (b) Z′ (or −Z′′) vs. ω^−1/2 plots and the linear fitting of carbon-coated LMP-3.0 in the Warburg region.

Table 2 Fitting results of the Nyquist plots using the equivalent circuit and the D_Li values

Sample	Re (Ω)	Ri (Ω)	Q1		Rct (Ω)	Q2		DLi (cm^2 s^−1)
			Y	n		Y	n
LMP-3.0	3.2	88.5	9.3 × 10^−5	0.58	25.4	1.6 × 10^−5	0.77	5.0 × 10^−15
LMP-3.5	2.2	84.8	6.5 × 10^−4	0.54	93.4	2.2 × 10^−5	0.68	1.6 × 10^−15
LMP-4.0	2.7	84.7	5.9 × 10^−4	0.58	117.3	2.1 × 10^−5	0.67	2.9 × 10^−15

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Li-ion chemical diffusion coefficients D_Li were also measured using EIS to further understand the different electrochemical behaviors between these LiMnPO₄/C samples. To calculate the D_Li values using the EIS technique, the Warburg factor σ in the Warburg region should first be determined. Fig. 8b shows the Nyquist plot of LMP-3.0 with marked frequencies f and the Warburg region with a slope of ∼45°. The inset in Fig. 8b correlates Z′ (or −Z′′) with ω^−1/2 (ω = 2πf) where σ can be obtained by linearly fitting the Z′ (or −Z′′) vs. ω^−1/2 plots. Thus, D_Li (cm² s⁻¹) values can be calculated using the following equation:^33,53,54

D_Li = R²T²/(2A²n⁴F⁴C²σ²) (1)

where R is the gas constant, T is the absolute temperature (K), A is the surface area of the electrode (cm²), n is the number of transferred electrons per LiMnPO₄ molecule upon complete delithiation, F is the Faraday constant, C is the Li-ion concentration in LiMnPO₄ (0.022 mol cm⁻³), and σ is the Warburg factor (Ω Hz^1/2). The D_Li values of the three samples are listed in Table 2. We can see that LMP-3.0 has a higher D_Li value than the other samples, implying the quickest Li-ion bulk diffusion. This can explain its better rate capability and enhanced high-rate cycling stability. The EIS results are consistent with the different electrochemical behaviors of the three samples.

4. Conclusions

In summary, we proposed a facile solvothermal route to synthesize LiMnPO₄ nanocrystals by using CA as a surfactant. The morphology and size of LiMnPO₄ change greatly upon adding a small amount of CA, which realizes the conversion of spindle-like LiMnPO₄ of 200 nm into granule-like LiMnPO₄ of 30–50 nm. The LiMnPO₄ granules display a superior rate capability and cycling stability at high rates after coating with a uniform/thin carbon layer of ∼1 nm thickness. At 20C, a high discharge capacity of 96.6 mA h g⁻¹ can be achieved for LiMnPO₄/C. The excellent rate capability is attributed to the small size with easy Li-ion transport at the electrode/electrolyte interface and in the bulk material, and to the uniform/thin carbon layer with enhanced electron transport. The LiMnPO₄/C granules also show outstanding high-rate cycling stability, with a discharge capacity of 80.5 mA h g⁻¹ maintained after 500 cycles at 10C. The long cycle life is ascribed to the small size which alleviates the lattice strain, and the uniform carbon coating which prevents Mn dissolution. The superior electrochemical performance of LiMnPO₄/C makes it promising for application in EVs.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2013CB934001), the National Natural Science Foundation of China (No. 51572238), Zhejiang Provincial Natural Science Foundation of China under Grant No. LY15E010004, and the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21264b

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